How does a hot-air balloon rise? Fire up the burner and the temperature of the air captured within the balloon increased. The molecules of the air take on the thermal energy and become more excited. The excited molecules create more space between them. As a result, the air expands as its temperature increases. Thus, one pound of air at 80-degrees Fahrenheit (°F) is a larger volume than one pound of air at 60°F, which is a larger volume than one pound of air at 40°F.
Moreover, air always has some amount of moisture in it. Proportionately, it is a very small amount of moisture relative to the overall air mass, but it is in there.
Also, air can only hold so much moisture. As air gets warmer and expands, it has the ability to hold more moisture. As the air temperature drops, it has less capacity to hold moisture.